1. Field of the Invention
The invention relates to network systems, and more particularly, to a system and method of serial wrap-around redundancy.
2. Description of the Prior Art
Computer networks are communications systems that allow multiple computers and devices to be interconnected by communications channels that facilitate communications and allow sharing of resources and information among interconnected devices. The information that is shared over a computer network may be represented in a range of logical representations that differ in complexity. The Open Systems Interconnection model (OSI model) is a way of sub-dividing a communications system into smaller parts called layers. Similar communication functions are grouped into logical layers. For example, the lowest layer, known as the physical layer, defines electrical and physical specifications for devices. In particular, the physical layer defines the relationship between a device and a transmission medium, such as a copper or optical cable.
One particular physical layer standard is known as RS-485. RS-485 is a Telecommunications Industry Association (TIA)/Electronics Industry Association (EIA) standard for multipoint communications and defines the electrical characteristics of drivers and receivers for use in balanced digital multipoint systems. RS-485 is formally known as TIA/EIA-485-A (2003) and is incorporated herein by reference in its entirety. RS-485 communication is known as half-duplex, meaning that a data source device either “talks” (transmits) or “listens” (receives/monitors) a signal, but does not do both at the same time. The RS-485 standard supports several types of connectors and enables configuration of inexpensive local networks and multipoint communications links. In a multi-drop configuration, one driver is connected to, and transmits on, a “bus” of receivers, where all of the receiver components are connected to a single set (e.g., a twisted pair) of electrical wires or connectors. In multipoint networks, such as with RS-422 communication for example, multiple drivers and receivers are each connected on a single bus where any node can simultaneously transmit or receive data over multiple sets of electrical wires or connectors (e.g., two twisted pairs of wires). By utilizing a differential balanced line over twisted pair, RS-485 networks can span relatively large distances—up to 4,000 feet. The RS-485 technical standard specifies three connector pins: a first pin carries an inverting signal (−), a second pin carries a non-inverting signal (+), and a third pin carries a common reference signal or ground.
An RS-485 network is typically configured in a master-slave arrangement that includes at least one master data source device and a plurality of slave endpoint devices. Examples of data source devices include computers, network servers, and control devices having a computer processor and the ability to receive and transmit data. Endpoint devices include data loggers, access control panels and card readers, alarm system components, climate control panels, pay stations, sensors, additional computers, and the like. The data source device(s) may control, communicate data to, and/or receive data from each endpoint device. RS-485 communication enables multipoint connection of typically up to thirty-two transmitters and up to thirty-two receiver devices.
RS-485 networks configured to have redundancy have at least two master data source devices. As illustrated in FIG. 1, a traditional single-string redundant RS-485 network 20 includes a first data source device 22, a second data source device 24, and a plurality of endpoint devices 26a-26d. Each endpoint device contains two endpoint interfaces 28, 30, one for each data source device 22, 24. Each endpoint interface 28, 30 is electrically connected to a data source device. Data redundancy is provided to each endpoint 26 on the RS-485 network 20 by duplicating hardware (i.e. cabling, and endpoint interfaces) that creates separate communication paths for endpoint data.
Another technical standard is the RS-422 standard, which can communicate in full-duplex or half-duplex modes. RS-422 is formally known as TIA/EIA-422-B (2000) and is incorporated herein by reference in its entirety. A full-duplex communication protocol uses four signal pins (two inverting (−), two non-inverting (+)) plus a ground pin. In RS-422 communication, a data source device may both transmit and receive at the same time due to the extra set of pins that carry additional inverting and non-inverting signals. Thus, one pair of pins carries a transmit signal while the other pair of pins carries a receive signal. This is different from RS-485 communication, in which a data source device can either transmit or receive, but cannot do both at the same time. RS-422 communications enable multipoint configuration of typically up to five transmitters and up to ten receiver devices.
For some applications, endpoints may be located in different spaces and floors of a factory, building, or ship where the use of a single string may not be practical. In these arrangements multiple strings are utilized in order to manage overall cable lengths. For example, a traditional multi-string redundant RS-485 network 40, as seen in FIG. 2, includes a first data source device 42, a second data source device 44, and a plurality of endpoint devices 46a-46h. Each endpoint device contains two endpoint interfaces 48, 50, one for each data source device 42, 44 with each endpoint interface 48, 50 being electrically connected to a data source device. Endpoint devices 46a-46d (e.g., located on a first floor of a building) are connected to first data source device 42 via interfaces 48a-48d, respectively, as part of a first string 43a. Endpoint devices 46a-46d are also connected to second data source device 44 via interfaces 50a-50d, respectively, as part of a second string 43b. Similarly, endpoint devices 46e-46h (e.g., located on a second floor of a building) are connected to first data source device 42 via interfaces 48e-48h as part of a third string 45a. Endpoint devices 46e-46h are also connected to second data source device 44 via interfaces 50e-50h as part of a fourth string 45b. 